Class ClassValue<T>

Lazily associate a computed value with (potentially) every type.
For example, if a dynamic language needs to construct a message dispatch
table for each class encountered at a message send call site,
it can use a ClassValue to cache information needed to
perform the message send quickly, for each class encountered.

get

Returns the value for the given class.
If no value has yet been computed, it is obtained by
an invocation of the computeValue method.

The actual installation of the value on the class
is performed atomically.
At that point, if several racing threads have
computed values, one is chosen, and returned to
all the racing threads.

The type parameter is typically a class, but it may be any type,
such as an interface, a primitive type (like int.class), or void.class.

In the absence of remove calls, a class value has a simple
state diagram: uninitialized and initialized.
When remove calls are made,
the rules for value observation are more complex.
See the documentation for remove for more information.

Parameters:

type - the type whose class value must be computed or retrieved

Returns:

the current value associated with this ClassValue, for the given class or interface

remove

Removes the associated value for the given class.
If this value is subsequently read for the same class,
its value will be reinitialized by invoking its computeValue method.
This may result in an additional invocation of the
computeValue method for the given class.

In order to explain the interaction between get and remove calls,
we must model the state transitions of a class value to take into account
the alternation between uninitialized and initialized states.
To do this, number these states sequentially from zero, and note that
uninitialized (or removed) states are numbered with even numbers,
while initialized (or re-initialized) states have odd numbers.

When a thread T removes a class value in state 2N,
nothing happens, since the class value is already uninitialized.
Otherwise, the state is advanced atomically to 2N+1.

When a thread T queries a class value in state 2N,
the thread first attempts to initialize the class value to state 2N+1
by invoking computeValue and installing the resulting value.

When T attempts to install the newly computed value,
if the state is still at 2N, the class value will be initialized
with the computed value, advancing it to state 2N+1.

Otherwise, whether the new state is even or odd,
T will discard the newly computed value
and retry the get operation.

Discarding and retrying is an important proviso,
since otherwise T could potentially install
a disastrously stale value. For example:

T calls CV.get(C) and sees state 2N

T quickly computes a time-dependent value V0 and gets ready to install it

T is hit by an unlucky paging or scheduling event, and goes to sleep for a long time

...meanwhile, T2 also calls CV.get(C) and sees state 2N

T2 quickly computes a similar time-dependent value V1 and installs it on CV.get(C)

T2 (or a third thread) then calls CV.remove(C), undoing T2's work

the previous actions of T2 are repeated several times

also, the relevant computed values change over time: V1, V2, ...

...meanwhile, T wakes up and attempts to install V0; this must fail

We can assume in the above scenario that CV.computeValue uses locks to properly
observe the time-dependent states as it computes V1, etc.
This does not remove the threat of a stale value, since there is a window of time
between the return of computeValue in T and the installation
of the the new value. No user synchronization is possible during this time.